Steel Buildings in Europe

Part 7: Fire Engineering 7 - 22 The critical temperature of the member can be calculated from the degree of utilization 0  as follows: 1 482 0.9674 1 39,19 ln 3.833 0            cr (11) The degree of utilization 0  is obtained from: fi,d,0 fi,d 0 R E   (12) where: fi,d E is the design effect of actions for the fire design situation, according to EN 1991-1-2 fi,d,0 R is the corresponding design resistance of the steel member, for the fire design situation, at time t = 0 (at normal temperature) but with safety factor M,fi  in fire situation The expression for θ cr can be used for all classes of section except the very slender Class 4 sections, for which a single conservative critical temperature of 350°C should be used. In principle, Expression (11) applies for members in pure bending, short columns without buckling and members in tension, heated uniformly or with slight temperature gradient. However, in situations of instability (slender columns, unrestrained beams), the method becomes applicable by calculating the design resistance for the fire design situation at time t = 0 with a value of the slenderness that takes into account temperature effects on the slenderness of structural members. As a simplification, the slenderness in fire situations can be taken as    1.3  (where  is the non dimensional slenderness at normal temperature). As an alternative, to relation (11) nationally determined critical temperatures can be given in the National Annex to EN 1993-1-2. A simple conservative expression for  0 can also be used for tension members and restrained beams (where lateral-torsional buckling is not a potential failure mode): 1 2 M M,fi fi,t 0        (13) where: fi,t  is the load level at time t M,fi  is the relevant partial safety factor for fire situation ( 1 M,fi   ) M0  is the partial safety factor at normal temperature ( 1 M0   )